1. Field of the Invention
The present invention relates to a nozzle for the turbine of a turbomachine such as an airplane turboprop or turbojet.
2. Description of the Related Art
A turbomachine turbine comprises a plurality of stages each having a bladed wheel and a nozzle. Each bladed wheel comprises a disk carrying substantially radial blades on its outer periphery, the disks of the various wheels being connected to one another along a common axis and being connected to a drive shaft of the turbine rotor by appropriate means.
Each nozzle comprises an inner annular platform and an outer annular platform with substantially radial vanes extending between them. The outer platform of the nozzle includes means for engaging and fastening to an outer casing of the turbine. The inner annular platform carries annular elements of abradable material for co-operating with outer annular wipers of a rotor element so as to form a labyrinth type seal. The seal serves to ensure that the flow of air passes axially through the annular space between the inner periphery of the nozzle and the rotor of the turbine.
In the present state of the art, the abradable material elements are carried by an annular ring that is connected to the inner platform of the nozzle by a radial wall. The ring, the radial wall, and the inner platform of the nozzle are formed as a single casting having considerable thicknesses and thus of relatively high weight.
That known technique presents another major drawback. In operation, the nozzle vanes are exposed to hot gas flowing in the flow section of the turbine. The temperature of the gas in the flow section is relatively high, typically of the order of 900° C., while the temperature in the zone between the inner platform of the nozzle and the rotor is lower, e.g. about 700° C. The thickness of the platform and of the means supporting the abradable elements cause them to present considerable thermal inertia which, combined with the lower temperature to which they are subjected, opposes thermal expansion of the vanes of the nozzle, which values are then subjected to high levels of mechanical stress. This gives rise to the appearance of cracks and fissures in the vanes causing their lifetime to be significantly reduced.
Proposals have already been made to fasten the abradable elements on pieces of sheet metal that are themselves fastened to an inner radial rim of the inner platform by fastener means of nut-and-bolt or analogous type. Unfortunately, such fastener means occupy a relatively large amount of axial and radial space, and using them significantly increases the size and weight of the nozzle.